1. Field of the Invention
The present invention relates to intravital-information imaging apparatuses.
2. Description of the Related Art
Generally, many imaging apparatuses that employ X-rays, ultrasound, or magnetic resonance imaging (MRI) are used in the medical field.
Furthermore, in the medical field, research is being conducted for optical imaging apparatuses for obtaining intravital information by irradiating a living body with light from a light source, such as a laser, so that the light propagates in the living body, and detecting the propagated light or the like.
As a type of optical imaging technique, photoacoustic tomography (PAT) has been proposed. Photoacoustic tomography is described, for example, in M. Xu and L. V. Wang, “Photoacoustic imaging in biomedicine”, Review of Scientific Instruments, 77, 041101 (2006).
In photoacoustic tomography, a living body is irradiated with pulsed light emitted from a light source, an acoustic wave generated from a biological tissue having absorbed the energy of light propagated and diffused in the living body is detected at multiple points, and corresponding signals are analyzed to form a visual image representing intravital information. This makes it possible to obtain a distribution of optical characteristic values in the living body, particularly, a distribution of optical-energy absorption densities.
According to the reference mentioned above, in photoacoustic tomography, the sound pressure (P) of an acoustic wave generated from a light absorber in a living body in response to absorbed light can be expressed by equation (1) below:P=Γ·μa·Φ  (1)where Γ denotes the Grüneisen coefficient, which is a value of elasticity characteristic determined by dividing the product of the thermal coefficient of volume expansion or isobaric volume expansion coefficient (β) and the square of the speed of light (c) by the specific heat at constant pressure (Cp).
μa denotes the absorption coefficient of the light absorber, and Φ denotes the amount of local light indicating the optical fluence (light with which the light absorber is irradiated).
Since it is known that Γ is substantially constant for a specific tissue, the distribution of the products of μa and Φ, i.e., the distribution of optical-energy absorption densities, can be obtained by measuring change in sound pressure P representing the magnitude of the acoustic wave at multiple points by time division.
In the photoacoustic tomography according to the related art described above, as will be understood from equation (1), in order to obtain a distribution of absorption coefficients (μa) in a living body from a measurement of change in sound pressure (P), it is necessary to obtain by some method the distribution of the local amounts of light with which a light absorber is irradiated. However, since light introduced in a living body is diffused, it is difficult to estimate the local amount of light with which the light absorber is irradiated. Thus, with ordinary measurement of sound pressures of a generated acoustic wave alone, it is only possible to make an image representing a distribution of optical-energy absorption densities (μa×Φ).
That is, it is difficult to calculate a distribution of the amounts of light with which the light absorber is irradiated (Φ) and to accurately separate and generate an image of a distribution of absorption coefficients (μa) in a living body on the basis of a measurement of sound pressures of an acoustic wave.
As a result, it is difficult to accurately identify constituents of biological tissues or to measure density on the basis of the distribution of absorption coefficients (μa) in a living body.